FIGURE 4.

(A-bottom left) Nervi conarii. The nervi conarii form an intermingling plexus of fibres over the pineal gland and release norepinephrine onto pinealocytes during the night to stimulate melatonin synthesis. (B-top right) Pinealocyte clusters. Spontaneously firing pinealocytes (Sp) are arranged into “clusters” of regularly firing cells (REG) and rhythmically firing cells (RHY). Clusters are surrounded by “silent” cells (Si) which exhibit no spontaneous firing. The firing of RHY cluster is synaptically linked to the firing of another (red arrows). The electrical response of pinealocytes is complex. Inhibitory and excitatory responses have been recorded from pinealocytes (+ and –). Specific groups of pinealocyte may become excited or inhibited depending upon the source of innervation i.e., left, right or both SCG. Regulation of pineal electrical responses might be also mediated by pinealocytes themselves, with the excitation of one pinealocyte causing the inhibition of another (green arrow).